The present invention relates to an oxide cathode for an electron tube such as a cathode ray tube, and more particularly, to a novel oxide cathode having an improved electron emission characteristic and long lifetime.
As a conventional thermoelectron emissive cathode for an electron gun of an electron tube, an oxide cathode having a carbonate of an alkaline earth metal on a metal base of which the major component is Ni is widely used. The cathode is called an "oxide cathode" because the carbonate of the alkaline earth metal changes to an oxide during the process for manufacturing an electron tube.
The oxide cathode has the advantage of operating at relatively low temperature (700.degree..about.800.degree. C.) since it has a low work function. On the other hand, when electron emission density increases, raw material evaporates or melts by self-heating due to Joule heat because the material is a semiconductor and has high electrical conductance, which thus deteriorates the cathode. Moreover, an interlayer is/brined between the metal base and the oxide layer due to prolonged operation, which shortens cathode lifetime.
FIG. 1 illustrates a cross-sectional view of a conventional oxide cathode. The general oxide cathode is provided with a disk-type metal base 2, a cylindrical sleeve 3 which supports the metal base 2, a heater 4 placed in the sleeve for heating the cathode, and an electron emissive material layer 1 which contains alkaline earth metal oxide as a main component and is coated and formed on the metal base 2. That is, the oxide cathode is manufactured by closing one end of a hollow cylindrical sleeve 3 with a metal base 2, inserting a heater 4 in the sleeve 3 for heating the cathode, and forming an electron emissive material layer 1 of a mixture of at least two alkaline earth metal compounds on the surface of the metal base 2.
Among the elements, the metal base provided on the sleeve supports the electron emissive material layer. The metal base uses heat-resistant metal materials such as platinum, nickel, etc. and is made of an alloy containing at least one reducing agent to help reduction of the alkaline earth metal oxide layer formed on the surface thereof. As for the reducing agent, reducible metals such as W, Mg, Si, Zr, etc. are usually employed, and the amount added varies according to their reducibility. More than two can be simultaneously employed to improve the cathode characteristics.
The sleeve supports the metal base and holds the heater therein. Heat-resistant metals such as molybdenum, tantalum, tungsten, stainless steel, etc. are selected for the raw materials of the sleeve considering thermal characteristics such as heat conductance.
The heater is provided in the sleeve to heat the electron emissive material layer coated on the metal base to emit thermo-electrons through the metal base. The heater is made by coating metal wire such as tungsten with alumina to form an electrically insulative layer.
The electron emissive material layer which emits thermo-electrons is formed on the surface of the metal base and is usually made of an alkaline earth metal (Ba, Sr, Ca, etc.) oxide layer. The oxide layer is manufactured by coating a dispersion of alkaline earth metal carbonate on the metal base and heating under vacuum using the heater to change the carbonate to all alkaline earth metal oxide. The layer is partially reduced at a high temperature of 900.degree..about.1000.degree. C. to activate the alkaline earth metal oxide to impart the characteristics of a semiconductor.
As the alkaline earth metal oxide, BaO mixed with SrO and/or CaO gives better electron emission characteristics than the single oxide of BaO. The reason is generally regarded as follows. That is, Sr and Ca are classified in the same family with Ba in the Periodic Table, and Sr and Ca become the same divalent cation as Ba ions and occupy the spaces where Ba ions had been. At this time, the immediately surrounding environment is somewhat disturbed since the atomic radius of Sr or Ca is different from that of Ba, which endows the oxygen ions with a high electric potential and thus makes them unstable. This is easily activated during reduction under a high temperature treatment and results in an advantageous aging.
The reducing agents, such as Si, Mg, etc., contained in the metal base diffuses during the activation process and thereby move toward an interface of the electron emissive material layer of alkaline earth metal oxide with the metal base, and reacts with the alkaline earth metal oxide as the following reaction. The barium oxide contained the electron emissive material layer is reduced through the reaction with the reducing agent, such as Mg, Si, etc., in the metal base to produce free barium. The free barium is the source of the electron emission. EQU BaO+Mg.fwdarw.MgO+Ba.uparw. EQU 4BaO+Si.fwdarw.Ba.sub.2 SiO.sub.4 +2Ba.uparw.
As described above, free barium from BaO plays the role of all oxygen-deficient semiconductor and, ultimately, emission current of 0.5.about.0.8A/cm at the operation temperature of 700.degree..about.800.degree. C. is obtainable.
However, generally since the operation temperature of the oxide cathode is so high (about 750.degree. C. or more), Ba, St, Ca, etc. are evaporated due to the vaporization pressure and the electron emission capacity decreases over operating time.
Meanwhile, as shown in the reaction equations, during free barium production, the reducing agents in the metal base also oxidize to produce oxides such as MgO, Ba.sub.2 SiO.sub.4, etc. These kinds of metal oxides are electrically insulative and accumulate to form an interlayer at the interface of electron emissive material layer with metal base, which acts as a barrier. The thus-formed barrier produces joule heat which increases the operating temperature. This also interrupts the diffusion of the reducing agents such as Mg, Si, etc. and suppresses the production of free barium. Moreover, the interlayer disturbs the replenishment of the evaporated Ba, Sr or Ca and results in the shortening of the cathode lifetime. Since the interlayer has high resistance, the flow of the electron emissive current is interrupted.
That is to say, tier the conventional oxide cathode, free Ba is continuously produced at the thermoelectron emission temperature, which enables electron emission accompanying partial evaporation of the free Ba. If a large amount of free Ba evaporates and is consumed, the electron emission function of the cathode deteriorates abruptly, and the cathode operation ends immediately.
Among the various factors which determine the lifetime of a cathode, the reduction of the barium content accompanied by the cathode operation and the interlayer growth as described above are important factors. Hence, research for improving the cathode lifetime as well as electron emission capability by changing electron emissive material components or including specific compounds therein has been carried out.
U.S. Pat. No. 4,797,593 discloses a technique on all improvement of the electron emission characteristic and cathode lifetime by including rare earth metal oxides in an electron emissive material layer.
Since the oxide cathode can be advantageously manufactured and has good characteristics, much investigation into the oxide cathode is being carried out and the oxide cathode is widely used as an electron emission source. However, recently as large and fine electron tubes are required a cathode for an electron tube having enhanced characteristics of electron emission and a longer lifetime is needed. Accordingly, the conventional oxide cathode needs further improvements since they do not meet the requirements owing to the above-mentioned various problems.